http://www.tombstonetrading.com/
I have spent years searching for particular hard to find in MA firearms and it is not easy. The longest I looked was about 8-10 years.
Dealers will tell you that they will look but the simple fact is that they have so many customers in and out the door in a weeks time, all looking for firearms, there is no simple way for them to keep track. You have already done some leg work and have come up with nothing. How long do you think a dealer will look when he has been told over and over again somone will take it only to get "I don't have the $ right now" after finding it?
The only way you are likely to find this gun is:
1) Make a list of shops with online inventory. Check it daily
2) Check classifieds Daily Post a WTB (if you have not already)
3) Hit some premium dealers (like Flotter) who may keep this in that back of their mind and give you a call if they come across one.
4) Hit local shops
5) Buy the one in black and have it refinished.
6) If someone has a MK IXX (19) I think the frame is all the same. Buy it in that cal and then get the parts to convert to .50AE.
7) Have the cash ready to spend and be prepared to spend more than the gun is worth (MA Premium).
I think Four Seasons has one in .44 and that frame may be the same as the .50AE depending on the YOM. I know early guns had different frames than later guns. Later guns, if I am correct, change cals with the barrel, and bolt where early guns may requre slide, barrel and bolt changes.
Also I do not know if you have shot one or not? If you have not, then I would try and shoot one or 2 to get a feel. The gun is a range toy, a fun range toy, but a range toy 50AE can be hard to find and expensive, alteritive cals may give you more enjoyment as well.
Good luck with the search.